-When will you arrive in this city again? - In a month.
I was always taught that we don't use the future simple after "with", but I came across with this sentence in my textbooks. So i'm confused why we don't use the present simple in this example?
According to traditional grammars (for example, see in the Oxford Living Dictionary https://en.oxforddictionaries.com/):
The Present Tenses are used:
'' to refer to a future situation in certain cases and in some subordinate clauses
(the bus arrives in London at 6 p.m.;
I'll make us some coffee when we get home)''.
The Future Simple Tense is not used in the subordinate clause of time (beginning with 'when', 'after', 'before', etc.),
but the main clause in this complex sentence has the predicate in the Future Simple Tense:
''When the parents arrive, l'll call you''.
In your case the sentence is simple.
It can have any verb form (including the Future Simple Tense).
There's another case when the Future Simple Tense can be used after 'when'.
This is the relative clause: ''He asked me when the parents will arrive''.